Encapsulation for particle entrapment

ABSTRACT

A packaged micromechanical device ( 100 ) having a blocking material ( 116 ) encapsulating debris-generating regions thereof. The blocking material ( 116 ) prevents the generation of debris that could interfere with the operation of the micromechanical device ( 100 ). Debris-generating regions of the device ( 100 ), including debris-creating sidewalls and any debris-harboring cavities, as well as electrical connections ( 108 ) linking the device ( 100 ) to the package substrate ( 102 ) are encapsulated by the blocking material ( 116 ). The blocking material ( 116 ) avoids contact with any debris-intolerant regions ( 118 ) of the device ( 100 ). A package lid ( 124 ), which is glass in the case of many DMD packages, seals the device ( 100 ) in a package cavity ( 120 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of application Ser. No. 09/705,466,filed Nov. 3, 2000 now U.S. Pat. No. 6,841,412 and ProvisionalApplication No. 60/163,862, filed Nov. 5, 1999.

The following patents and/or commonly assigned patent applications arehereby incorporated herein by reference:

Patent No. Filing Date Issue Date Title 6,147,790 May 13, 1999 Nov. 14,2000 Spring-Ring Micromechanical Device

FIELD OF THE INVENTION

This invention relates to the manufacture of micromechanical devices,more particularly to packaging of micromechanical devices such asmicromirror devices.

BACKGROUND OF THE INVENTION

Micromechanical devices are small structures typically fabricated on asemiconductor wafer using techniques such as optical lithography,doping, metal sputtering, oxide deposition, and plasma etching similarto those developed for the fabrication of integrated circuits.

Digital micromirror devices (DMDs), sometimes referred to as deformablemicromirror devices, are a type of micromechanical device. Other typesof micromechanical devices include accelerometers, pressure and flowsensors, gears and motors. While some micromechanical devices, such aspressure sensors, flow sensors, and DMDs have found commercial success,other types have not yet become commercially viable.

One problem common to most micromechanical devices is the problem ofparticulate contamination. While some micromechanical devices, such aspressure sensors, may not have exposed moving parts, most devices dohave exposed components that move relative to each other. These movingparts can become mechanically blocked or electrically shorted by verysmall particles of debris. Although steps are taken during manufactureof the devices to clean debris from the devices, the fragile nature ofsome micromechanical devices and other factors conspire to prevent totalelimination of the debris. Thus, debris inside micromechanical packagescontinues to be a significant cause of mechanical and electricalfailures. What is needed is a way to prevent these failures caused bydebris inside the micromechanical device package.

SUMMARY OF THE INVENTION

Objects and advantages will be obvious, and will in part appearhereinafter and will be accomplished by the present invention whichprovides a method and system for encapsulating debris-generating regionsof the micromechanical devices. One embodiment of the claimed inventionprovides a method of protecting debris-intolerant micromechanicaldevices. The method comprises attaching a device to a substrate,encapsulating at least one debris-generating region with a blockingmaterial, while avoiding contact between the blocking material and anydebris-intolerant regions.

According to another embodiment of the claimed invention, a packagedmicromechanical device is provided. The packaged micromechanical devicecomprises a package substrate, a micromechanical device supported by thepackage substrate and having at least one debris-generating region,blocking material attached to the device and covering at least onedebris-generating region, and a package lid supported by the packagesubstrate and enclosing the micromechanical device and the blockingmaterial.

Advantages of the claimed method and system include the permanentblocking of debris generated by the debris-generating portions of thedevice. The debris-generating portions of the device includedebris-creating regions such as the device sidewalls, anddebris-harboring regions such as cavities under the device and otherportions of the device, including the sidewall, that can trap debris andlater release it. The blocking material not only provides permanententrapment of debris, but entrapment of the debris is often cheaper thanmethods of eliminating the existing debris, such as cleansing orscrubbing the devices. While scrubbing the device can remove looseparticles that have not yet broken off the device, scrubbing cannotprevent the device from creating particles in the future. An additionalbenefit of the blocking material is due to the reduction in packagecavity volume. Since the blocking material displaces the gasses normallycaptured in the cavity, the environmental effects on the packaged deviceare reduced since there is less gas to react with the device.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of a micromechanical device, such as adigital micromirror device (DMD™), mounted to a package substrate.

FIG. 2 is a cross-section side view of the device and package substrateof FIG. 1 showing bond wires connecting the bond pads on the device tothose on the package substrate.

FIG. 3 is a cross-section side view of the device of FIG. 2 showing aquantity of blocking material deposited around and under the device.

FIG. 4 is a cross-section view of the device of FIG. 3 showing theblocking material covering not only the sidewalls and debris-harboringregions of the device, but the bond wires as well.

FIG. 5 is a schematic side view showing a typical application methodusing a syringe and two moveable X–Y stages.

FIG. 6 is a cross-section side view of a device completely enclosed byblocking material.

FIG. 7 is a cross-section side view of the device of FIG. 6 having aportion of the blocking material removed to expose debris-sensitiveregions of the device.

FIG. 8 is a cross-section side view of the device of FIG. 4 sealed in apackage cavity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A new system and method has been developed to eliminate many of thedebris-caused failures of micromechanical devices. These failures areeliminated by encapsulating the debris such that it cannot migrate todebris sensitive portions of the micromechanical device. The debris isencapsulated in place over portions of the device that are tolerant ofthe encapsulating material. The encapsulating material avoids regions ofthe micromechanical device, such as exposed moving components, thatwould be harmed by contact with the encapsulating material. In additionto encapsulating existing debris, the encapsulating material is alsodeposited over regions of the device that have been determined togenerate significant quantities of debris. These regions typicallyinclude the sidewalls of the silicon substrates on which manymicromechanical devices are formed.

FIG. 1 is a perspective view of a micromechanical device 100 such as adigital micromirror device (DMD™) mounted to a package substrate 102.FIG. 1, like the rest of the figures shown, is not drawn to scale but isdrawing to illustrate particular features relevant to the inventiondisclosed herein. The device 100 in FIG. 1 is a portion of a siliconwafer on which a micromechanical device has been formed. The device hasa debris intolerant area 104, which in the case of a DMD includes thearray of mirrors. The perimeter of the device, and a region of thepackage surrounding the device, includes many bond pads 106 that allowelectrical connection between the device and the package.

FIG. 2 is a cross-section side view of the device 100 and packagesubstrate 102 of FIG. 1 showing bond wires 108 connecting the bond pads106 on the device to those on the package substrate 102. Two regions ofthe device shown in FIG. 2 tend to be debris-generators. First, the sideof the device, or sidewall 110, tends to create debris throughout thelife of the device. The sidewall 110 is the portion of the device thatis fractured when the device is separated from a silicon wafer. Theseparation process, whether performed by breaking the wafer or sawingthe wafer, creates many fissures in the silicon crystal wafer. As thedevice undergoes thermal cycles and vibration, these fissures can growand release silicon debris. Thus, even if all of the debris is washedfrom the device after wafer separation, the device itself can generateadditional debris even after the device is hermetically sealed in apackage.

The second class of debris-generating regions trap, and later release,debris. For example, debris can be trapped under the device in a gap 112formed by voids in the adhesive 114 used to attach the device 100 to thepackage substrate 102, or in other regions that are difficult to rinsedebris out of.

According to one aspect of the present invention, a blocking material isdeposited on either the debris generating, or debris harboring regions,or both, to prevent debris from being released or generated by thedevice and migrating to the debris-intolerant portions of the device.Typically, the blocking material is an adhesive that is deposited aroundthe perimeter of the device after the device is attached to the packagesubstrate.

FIG. 3 is a cross-section side view of the device of FIG. 2 showing aquantity of blocking material 116 that has been deposited around andunder the device 100. As shown in FIG. 3, the blocking material 116surrounds the device 100, encapsulating the debris-creating sidewallregions and either filling the debris-harboring regions or blockingdebris from exiting the debris-harboring regions 112. The blockingmaterial of FIG. 3 avoids the debris-sensitive regions 118 of the device100. In FIG. 3, the device 100 is a digital micromirror device and thedebris-sensitive region is the array of micromirrors on the surface ofthe device 100.

FIG. 4 is a cross-section view of the device 100 of FIG. 3 showing theblocking material 116 covering not only the sidewalls anddebris-harboring regions of the device 100, but the bond wires as well.Encapsulating the bond wires is sometimes necessary in order to coverthe sidewalls 110 of a device 100 after the device 100 has been bondedto the package substrate 102 and electrically connected using bond wires108. Encapsulating the bond wires 108 not only is necessary, it mayimprove operation of the device in a high-vibration environment.

The ideal blocking material should be easy to apply to the device 100with sufficient control to ensure coverage of the debris-creating andharboring regions without allowing the blocking material 116 to reachthe debris-sensitive regions. Additionally, the blocking material 116should adhere to the device and other surfaces sufficiently to preventthe material from coming free of the device surface. One family ofcompounds that is ideally suited for use as a blocking material is thefamily of photo-curable adhesives. Two particularly well-suitedadhesives are NOA88 and NEA123, both UV-curable optically-clearadhesives produced by Norland Corporation. While NOA88 and NEA123 hardenwhen they are cured, compounds that remain sticky could provide agettering function inside the package as well as a blocking function.

When used with digital micromirror devices, which must be undercut toremove the photoresist on which the micromirrors are formed, the devices100 are typically undercut prior to bonding the device 100 to a packagesubstrate 102. If the process flow uses the recently-developed method ofattaching the device 100 to the package substrate 102 prior toundercutting the micromirrors, the blocking material may be appliedprior to, or after, the mirrors are undercut. Preferably, the blockingmaterial is applied prior to undercutting the micromirrors. In thismanner, the debris is prevented before the micromirrors become sensitiveto the debris. If the blocking material is applied before theundercutting process—a plasma etch process—the blocking material must beable to withstand the undercutting process without significantdegradation.

The blocking material is applied in a variety of ways. The typicalapplication method uses a syringe 120, shown in FIG. 5, to expelblocking material while the syringe 120 and device 100 are movedrelative to each other, typically by use of an X–Y stage 122. Thesyringe 120 can be either a positive displacement or a pneumaticsyringe. The amount of blocking material deposited is controlled by therate of movement of the syringe 120 plunger and the rate of relativemotion generated by the X–Y stage. Examples of suitable syringes 120include positive displacement syringes, in which a motor controlsmovement of the plunger, and pneumatic syringes, in which the plunger iscontrolled by air pressure. Many other application methods, such asspraying or dipping, can be used without departing from the teachings ofthis disclosure.

A second method of applying the blocking material is shown in FIGS. 6and 7. In FIG. 6, the blocking material completely encloses or pots thedevice. A portion of the blocking material 116 is then removed, as shownin FIG. 7, to expose the debris-sensitive regions 118 of the device 100.Selective removal of the blocking material 116 is accomplished byphoto-lithographic processes using masks, by selectively curing part ofthe blocking material and removing the uncured material, or through theuse of other means. While the encapsulate and remove process requiresadditional steps and is therefore expected to be more cumbersome andexpensive, it can provide very precise location of the blocking materialif necessary. For example, the blocking material can be removed to forman aperture over an optically active region of the device.

FIG. 8 is a cross-section side view of a packaged micromechanical device100 showing use of blocking material 116 to prevent the generation ofdebris that could interfere with the operation of the micromechanicaldevice 100. In FIG. 8, the debris-generating regions of the device 100,including the debris-creating sidewalls and any debris-harboringcavities, as well as the electrical connections 108 linking the device100 to the package substrate 102 are encapsulated by the blockingmaterial 116. The blocking material 116 avoids contact with anydebris-intolerant regions 118 of the device 100. A package lid 124,which is glass in the case of many DMD packages, seals the device 100 ina package cavity 120.

The blocking material 116 not only provides permanent entrapment ofdebris, including debris generated after the device is packaged, butentrapment of the debris is often cheaper than methods of eliminatingthe existing debris, such as scrubbing the devices. While scrubbing thedevice will remove loose particles that have not yet broken off thedevice, scrubbing cannot prevent the device from generating particles inthe future. An additional benefit of the blocking material 116 is thereduction in package cavity volume when the blocking material 116 isused. Since the cavity volume is reduced, the environmental effects onthe packaged device are also reduced since there are fewer capturedgasses to react with the device.

Thus, although there has been disclosed to this point a particularembodiment of sidewall encapsulation and method therefore, it is notintended that such specific references be considered as limitations uponthe scope of this invention except insofar as set forth in the followingclaims. Furthermore, having described the invention in connection withcertain specific embodiments thereof, it is to be understood thatfurther modifications may now suggest themselves to those skilled in theart, it is intended to cover all such modifications as fall within thescope of the appended claims.

1. A packaged micromechanical device comprising: a package substrate; amicromechanical device supported by said package substrate, saidmicromechanical device having at least one debris-generating region;blocking material attached to said device and covering at least one saiddebris-generating region, said blocking material avoiding contact with afirst region of said device wherein said first region is either adebris-intolerant region or an optically active region; and a packagelid supported by said package substrate and enclosing saidmicromechanical device and said blocking material.
 2. The device ofclaim 1, at least one said debris-generating regions comprising asidewall formed where said device was attached to a wafer.
 3. The deviceof claim 1, said blocking material comprising an adhesive.
 4. The deviceof claim 1, said blocking material comprising an photo-curable adhesive.5. The device of claim 1, said blocking material comprising an adhesivethat remains tacky to perform a gettering function.
 6. The device ofclaim 1, further comprising: electrical connections between said deviceand said package substrate, said blocking material encapsulating saidelectrical connections.
 7. A packaged micromechanical device comprising:a package substrate; a micromechanical device supported by said packagesubstrate, said micromechanical device having at least onedebris-generating region; blocking material attached to said device andcovering at least one said debris-generating region, said blockingmaterial avoiding contact with a first region of said device and havinga tacky surface when cured; and a package lid supported by said packagesubstrate and enclosing said micromechanical device and said blockingmaterial.
 8. The device of claim 7, at least one said debris-generatingregions comprising a sidewall formed where said device was attached to awafer.
 9. The device of claim 7, said blocking material comprising anadhesive.
 10. The device of claim 7, said blocking material comprisingan photo-curable adhesive.
 11. The device of claim 1, furthercomprising: electrical connections between said device and said packagesubstrate, said blocking material encapsulating said electricalconnections.